60 TEXT-BOOK OF PHYSIOLOGY 



When the current is sufficiently strong to elicit both making and breaking 

 contractions, it is found that the contraction occurring on the make or closure 

 of the circuit is regularly greater than that occurring on the break or opening 

 of the circuit. Moreover, it has been shown in many ways that the con- 

 traction occurring on the closure of the circuit has its origin at the point 

 where the current is leaving the muscle i.e., in the immediate neighborhood 

 of the negative pole or cathode and propagates itself to the opposite extrem- 

 ity; while the contraction occurring on the opening of the circuit has its 

 origin at the point where the current is entering the muscle, i.e., in the 

 neighborhood of the positive pole or anode. 



These facts can be readily demonstrated by destroying the irritability 

 and contractility of one extremity of a muscle with parallel fibers such as the 



FIG. 20. DIAGRAM TO SHOW THE EFFECT OF LOCAL INJURY ON THE IRRITABILITY OF A 

 MUSCLE. (After Starling.} C Z an electric cell from which wires pass to non-polarizable elec- 

 trodes, anode and kathode, hi contact with a muscle, the injured end of which is more deeply 

 shaded. The arrows indicate the direction of the current. 



sartorius. On applying non-polarizable electrodes to the muscle as in Fig. 

 20, A, it will be found that when the circuit is made a contraction occurs 

 which must, of course, have developed at the irritable cathodic region, for 

 on the break of the circuit the muscle remains at rest. When the electrodes 

 are applied as in Fig. 20, B, and the circuit made the muscle remains at 

 rest, but on the break of the circuit a contraction occurs which must have 

 developed at the irritable anodic region. 



The Induced Current. If the primary coil of the inductorium be con- 

 nected with an electric cell and the secondary coil be connected with a 

 muscle, it will be found that the current induced in the secondary circuit, 

 both on the make and break of the primary, will also cause the muscle to 

 pulsate sharply and rapidly if the two coils are sufficiently near each other. 

 Observation, however, makes it evident that the pulsation occurring with 

 the break of the primary circuit is more energetic than that occurring with 

 the make, a result the opposite of that obtained with the constant current. 

 This is not due to any difference in the electricity, however, but to peculiari- 

 ties in the construction of the inductorium. When the primary circuit is 

 interrupted with sufficient frequency, as it can be by throwing into the circuit 

 Neef's hammer or some other form of interrupter, the contractions excited 

 by the induced currents may be made to succeed one another so rapidly that 

 they become fused together, producing a spasm or tetanus of the muscle. 

 The rapidity with which the induced current appears and disappears, its 

 brief duration, the ease with which its strength can be regulated, combine 

 to render it a most efficient stimulus for either muscle or nerve. 



